Several soybean (Glycine max) germplasms, such as Nishiyamahitashi 98-5 (NH), have an intense seaweed-like flavor after cooking because of their high seed S-methylmethionine (SMM) content. In this study, we compared the amounts of amino acids in the phloem sap, leaves, pods, and seeds between NH and the common soybean cultivar Fukuyutaka. This revealed a comparably higher SMM content alongside a higher free Met content in NH seeds, suggesting that the SMM-hyperaccumulation phenotype of NH soybean was related to Met metabolism in seeds. To investigate the molecular mechanism behind SMM hyperaccumulation, we examined the phenotype-associated gene locus in NH plants. Analyses of the quantitative trait loci in segregated offspring of the cross between NH and the common soybean cultivar Williams 82 indicated that one locus on chromosome 10 explains 71.4% of SMM hyperaccumulation. Subsequent fine-mapping revealed that a transposon insertion into the intron of a gene, Glyma.10g172700, is associated with the SMM-hyperaccumulation phenotype. The Glyma.10g172700-encoded recombinant protein showed Met--lyase (MGL) activity in vitro, and the transposon-insertion mutation in NH efficiently suppressed Glyma.10g172700 expression in developing seeds. Exogenous administration of Met to sections of developing soybean seeds resulted in transient increases in Met levels, followed by continuous increases in SMM concentrations, which was likely caused by Met methyltransferase activity in the seeds. Accordingly, we propose that the SMM-hyperaccumulation phenotype is caused by suppressed MGL expression in developing soybean seeds, resulting in transient accumulation of Met, which is converted into SMM to avoid the harmful effects caused by excess free Met.

一些大豆 ( Glycine max ) 种质，如 Nishiyamahitashi 98-5 (NH)，由于其种子S-甲基甲硫氨酸 (SMM) 含量高，在烹饪后具有强烈的海藻样风味。在这项研究中，我们比较了 NH 和普通大豆品种 Fukuyutaka 的韧皮部汁液、叶子、豆荚和种子中的氨基酸含量。这揭示了 NH 种子中相对较高的 SMM 含量以及较高的游离 Met 含量，表明 NH 大豆的 SMM 超积累表型与种子中的 Met 代谢有关。为了研究 SMM 超积累背后的分子机制，我们检查了 NH 植物中的表型相关基因位点。对 NH 与普通大豆品种 Williams 82 杂交的分离后代的数量性状位点分析表明，10 号染色体上的一个位点解释了 71.4% 的 SMM 超积累。随后的精细作图表明，转座子插入基因Glyma.10g172700的内含子中与 SMM 超积累表型相关。 Glyma.10g172700编码的重组蛋白在体外表现出 Met-裂解酶 (MGL) 活性，并且 NH 中的转座子插入突变有效抑制了发育种子中的Glyma.10g172700表达。向发育中的大豆种子切片外源施用 Met 导致 Met 水平短暂增加，随后 SMM 浓度持续增加，这可能是由种子中 Met 甲基转移酶活性引起的。 因此，我们认为，SMM超积累表型是由发育中的大豆种子中MGL表达受到抑制引起的，导致Met短暂积累，将其转化为SMM以避免过量游离Met造成的有害影响。